Certain applications require a driver transistor with a low-on resistance. An n-channel transistor with a large width or W/L ratio can be used to reduce on-resistance, which is especially useful when high drain currents are required by the application. Power management and power driver applications often require such a device.
Electric motors require high currents to energize magnets to turn a metal rotor. An alternating current (A.C.) is applied to the motor to quickly reverse the direction of magnet fields to propel the rotor as it turns. Full-bridge and half-bridge circuits are used to drive electric motors. These motor bridge circuits are an example of an application requiring low on-resistance.
An on-resistance of half an ohm or less is desirable. While this is easy to achieve for low-side n-channel drivers that drive the motor to ground, the high-side drivers that drive the motor to high voltages such as the power supply voltage are more problematic. A p-channel transistor could be used for the high-side driver, but the lower hole mobility causes the on-resistance to double or triple compared with an n-channel transistor with the better electron mobility.
When an n-channel transistor is used as the high-side driver, the gate voltage is often driven to a boosted voltage. The boosted voltage can be generated from the supply voltage using a charge pump. The boosted voltage is above the supply voltage. An oscillator can drive a capacitor between two diodes in the charge pump to generate the boosted voltage as is well known in the art.
Unfortunately, the charge pump supplies a dynamic current to maintain the boosted voltage. Also, there is a dynamic current drain from the boosted voltage node as gates such as the high-side driver transistor are charged and discharged. The predriver and other circuits that control the high-side driver transistor can also draw both static and dynamic current from the charge pump when they are powered by the boosted voltage rather than the supply voltage. A large charge-pump capacitor is required to supply the current. Thus the charge pump capacitor cannot be integrated on chip.
What is desired is a motor driver circuit that reduces current draw from the charge pump. A high-side driver circuit is desired with a reduced current draw and power consumption from the boosted supply. It is desired to reduce the charge pump capacitance is for integration on chip. A motor driver circuit that more efficiently uses a bootstrapped supply is desirable. An integrated motor driver circuit is desired.